directive: at
Specify a time to perform a task that can only be performed once
syntax: # at [parameter] [TIME]
at> Execution of instructions
Exit at Command Ctrl+d
Command Prerequisites:
The ATD process needs to be turned on (ATD is started by default with system boot in Redhat)
The test found that there is no own at service in the centos6.5 mini version, so you need to download it yourself
# yum-y Install at
# #查看是否开启atd
# Ps-ef | grep ATD
# #开启atd
#/ETC/INIT.D/ATD Start
# #设置atd开机启动
# chkconfig--level 2345 ATD on
Query deletes the current pending task
Queries the current wait task and is not displayed after it is executed
# ATQ
Delete a task in the system that is being set up by an at and waiting to be executed
# ATRM task's work number
For example:# ATRM
Common parameters:
-M: When the specified task is completed, the user will be sent a message, even if there is no standard output
Aliases for-I:ATQ
Aliases for-D:ATRM
-V: Shows when the task will be executed
-C: Print the contents of the task to standard output
-V: Display version information
-Q: Add < queue after > use specified queue
-F: Append < file > read into task from specified file instead of reading from standard input
-T: Back < time parameters > submit a task to run in the form of a time parameter
Time: Defines when to perform the at task, in the following format:
1, hh:mm
Note: At today's hh:mm time, if the time has passed, tomorrow's hh:mm will do this task.
ex> 04:00
2, hh:mm YYYY-MM-DD
Description: Specifies that the task be carried out at a particular time of day of the month of the year
ex> 04:00 2009-03-17
3, HH:MM[AM|PM] [Month] [Date]
A provision for the task at some point in a certain month of the year
ex> 04pm March
4, HH:MM[AM|PM] + number [Minutes|hours|days|weeks]
Description: Specify how much time will be added to the task at a certain point in time.
Ex> now + 5 minutes
Ex> 04pm + 3 days
Time format extension:
At allows you to use a fairly complex set of time-specific methods.
1, can accept in the day hh:mm (Hours: minutes) type of time designation. If the time has passed, then it will be executed the next day.
2, can use Midnight (late night), noon (noon), Teatime (tea time, usually 4 o'clock in the afternoon) and other vague words to specify the time.
3, can use the 12-hour timekeeping system, that is, after the time plus AM (morning) or PM (afternoon) to indicate whether it is morning or afternoon.
4, can specify the specific date of the command execution, the format is Month Day (month) or mm/dd/yy (Month/day/year) or Dd.mm.yy (day. Month), the specified date must be followed by the specified time.
5, can use the relative timing method. The specified format is: Now + Count Time-units, today is the current time, Time-units is the time unit, here can be minutes (minutes), hours (hours), Days (day), Weeks (week). Count is the amount of time, days, hours.
6. Be able to specify the time to complete the command directly using today (present), Tomorrow (Tomorrow).
Restrict users ' use rights
Premise: Many hosts by the so-called attack, most often found that their system is a lot of hacking programs, these programs are likely to use a number of scheduled tasks to run or collect your system running information, and regularly sent to the hacker. So, unless it's your approved account, don't let them use the AT command first.
The control file used by the AT command to restrict user control of use
Control file directory:/etc/at.allow and/etc/at.deny
Control file Usage Rules:
1: First find/etc/at.allow This file, write in this file user can use at, no user in this file can not use at (even if not written in At.deny);
2: If/etc/at.allow does not exist, look for/etc/at.deny this file, if it is written in this At.deny user can not use at, but not in this At.deny file users will be able to use the AT command.
3: If none of the two files exist, then only root can use the AT command.
4: In general distributions, because it is assumed that all users on the system are trustworthy, the system usually retains an empty/etc/at.deny file, which means that all people are allowed to use the AT command.
5: If you do not want some users to use at, the user's account will be written to/etc/at.deny! One account to write a line.
Precautions
1, if the command output at the wrong path will send the results in the form of mail to the user
2, when a task is created will be assigned to a task number, and will be queued inside the/var/spool/at. It is not recommended to use VI Editor to modify, error-prone.
Example:
Example 1: Three days after 5 o'clock in the afternoon Chung execution of/bin/ls
# at 5pm + 3 days
At>/bin/ls
At> <EOT>
Job 7 at 2013-01-08 17:00
Example 2: Tomorrow 17 o'clock, the output time is within the specified file
# at 17:20 tomorrow
At> Date >/root/doiido.log
At> <EOT>
Job 8 at 2013-01-06 17:20
Example 3: After scheduling a task, use the ATQ command to see that the system is not performing a work task before it is executed
# ATQ
8 2013-01-06 17:20 a root
7 2013-01-08 17:00 a root
Example 4: Delete a task that has already been set
# ATQ
8 2013-01-06 17:20 a root
7 2013-01-08 17:00 a root
# ATRM 7
# ATQ
8 2013-01-06 17:20 a root
Example 5: Displaying the contents of a task that has been set
# At-c 8
#!/bin/sh
# Atrun Uid=0 gid=0
# Mail Root 0
echo "Hello"
Date > Doiido.log
Extended instruction Batch
Batch is a special version of the AT command that is used when performing tasks that consume a lot of resources, only when CPU requirements are less than 80% of CPU capacity
# Batch
at> echo "HI" >/dev/tty2
The tasks created by batch are also viewed through ATQ, atrm delete
Linux instruction detailed at timed execution task
